Detection apparatuses, for example, intrusion monitoring apparatuses, are well known within the art. Typically, they are used to detect unauthorised entry or intrusion into a protected volume.
Commercially available intrusion monitoring apparatuses can be either passive or active. Passive intrusion monitoring apparatuses can comprise a sensor which detects infrared radiation emitted by people. Typically, such passive apparatuses comprise a thermal detection apparatus consisting of one or more thermal sensors arranged to detect infrared radiation and an optical system for directing such infrared radiation towards the thermal sensors. The optical system comprises at least one lens formed from a plurality of Fresnel lenses or at least portions thereof. Each Fresnel lens of the plurality of lenses is typically known as a facet. Conventionally, facets view or monitor respective regions or angular sectors of the protected volume. Such apparatuses are activated when a source of infrared radiation passes from one region or angular sector to the next, that is, infrared radiation is detected in a plurality of angular sectors. Typical prior art intrusion monitoring apparatuses are illustrated in, for example, U.S. Pat. Nos. 3,703,718 and 3,958,118 and UK patent application number 1,335,410, the entire disclosures of which are incorporated herein by reference for all purposes.
Active intrusion monitoring apparatuses are also known which comprise a transmitter and a receiver. The transmitter emits radiation at a defined frequency and the receiver measures the Doppler shift in any reflected signal. Such active monitoring apparatuses can, for example, operate at microwave frequencies using a microwave detection apparatus to detect the reflected signal.
The above active and passive detection apparatuses can be used alone or in conjunction with one another. Apparatuses that use two or more technologies, that is, a passive detection technology and an active detection technology, to identify intrusion into a protected volume or, more particularly, movement of an intruder within the field of view of the apparatus, are known within the art as combined detectors, combined technology apparatuses, dual technology or multi-technology devices. Examples of combined detectors that use a photoelectric sensor and a microwave sensor are disclosed in U.S. Pat. Nos. 3,725,888 and 4,401,976, the entire disclosures of which are incorporated herein for all purposes by reference. There exists a British standard relating to combined passive infrared and microwave detectors, which is “Alarm systems-Intrusion systems-Part 2-4: Requirements for combined passive infrared and microwave detectors”, the content of which is incorporated herein by reference for all purposes.
However, the revised DD243-2004 standard, entitled “Installation and configuration of intruder alarm systems designed to generate confirmed alarm conditions—Code of practice”, under section 5.4, entitled “Design and configuration of sequential confirmation IASs”, provides that within a sequentially confirmed alarm the movement detectors are not allowed to overlap each other. Furthermore, section 5.4.2 states that “[therefore], movement detectors should be located some distance apart, generally with a minimum distance between detector housings of 2.5 m”. One skilled in the art clearly appreciates that the above is a costly solution to the problem of providing sequentially confirmed alarms since it requires twice the investment, that is, two detectors, twice the cabling etc.
In one typical combined technology device the outputs of two independent sensing means, that is, the photoelectric sensor and the microwave sensor, responding to different stimuli, must be present within a predetermined period of time to register an event, that is, intrusion by an intruder into the field of view or fields of view of the combined technology apparatus.
The European Committee for Electrotechnical Standardisation is responsible, amongst other things, for establishing technical standards relating to intrusion detection or detection apparatuses. For example, technical specification CLC/TS 50131-2-4:2004, entitled “Alarm systems-Intrusion Part 2-4: Requirements for combined passive infrared and microwave detectors”, establishes a base or minimum set of standards or tests to be achieved by microwave detectors. The microwave detectors are given a corresponding grade according to the number or level of tests they pass, that is, according to the degree to which they correspond to the technical specifications or the specifications established by the class of 50131 standards. The above technical specifications are incorporated, for all purposes, herein by reference. The technical specifications provide for a number of security grades; namely, security grades 1 to 4. A requirement of EN 50131-1:1997 is that grade 3 and 4 systems shall have detectors that are able to detect a significant reduction in range. It will be appreciated that EN 50131-2-4:2004 applies to grade 4 detectors only. A simulated walk test is used to determine whether or not a detector is worthy of a corresponding grade. Typically, when assessing detector performance, a detector should generate an intrusion signal or message when an SWT or simulated walk test target moves within and across the detector's claimed boundary of detection for a distance of 3 meters. The detector shall also generate an intrusion signal or message when the standard or simulated walk test target moves at velocities and attitudes that meet the requirements specified of the technical standard CLC/TS 50131-2-4:2004. It can be appreciated from section 4.2.3 of that standard that the requirement headed “Significant reduction of specified range” is such that grade ¾ detectors should be capable of detecting “a range reduction along [a] principal axis of detection of more than 50% within a maximum period of 180s according to the requirements of Table 2”. It will be appreciated that range reduction is discussed with reference to figure C.5 of that standard. Furthermore, it is indicated that the requirements of 4.3.5 (self test) and 4.5.5 (resistance to masking) can provide range reduction detection. Section 6.4.5, entitled “Verify the significant reduction of specified range” specifies a test to be met in determining whether or not a detector can detect a significant reduction of a specified range according to the technical specification. The test is as follows. A test point on a detector axis at a distance of 55% of the manufacturer's claimed detection range is selected. A barrier of cardboard boxes is erected across the axis such that it is normal, that is, perpendicular, to it at a distance of 45% of the manufacturer's claimed detection range. The barrier is such that it covers a horizontal distance of plus and minus 2.5 metres either side of the axis and has a vertical height of 3 metres such as is shown in figure C.5 of the technical specification CLC/TS 50131-2-4-2004. At the test point, two test directions are used, beginning at a distance of 1.5 metres before the test point, and finishing 1.5 metres after it, moving perpendicularly to the detector axis. The SWT shall move along each path from start to finish. At the end of each walk test, the SWT shall pause for at least 20 seconds before carrying out any further tests. The pass/fail criterion is such that an alarm or fault signal or message is generated when the barrier is present. It will be appreciated that a corresponding standard also prescribes requirements for passive infrared detectors; namely, DD CLC/TS 50131-2-2:2004.
In a further typical combined technology event detection device, the outputs of two independent sensing means, responding to different physical stimuli, are processed to determine if both sensing means register an event within a specified period of time, and, if so, an alarm is triggered. In this manner the incidence of false alarms occurring when only a single sensor means is used can be greatly reduced.
A problem with both single and combined technology event detection devices is that if the detector is masked, for example, by tampering with the outer casing of the detector, or by placing a screen in front of the detector which will absorb the microwave signals emitted by the microwave device, or which will block infra red signals and prevent them from reaching the passive infra red sensor, the event detection device is rendered inoperable.
Attempts have been made to overcome this problem by providing the event detection device with a separate system comprising an infra red LED emitter and a detector which operate at a frequency range different from that of the passive infra red sensor. If an object is placed near the event detection device so as to mask the passive infra red sensor, the infra red LED/detector system will detect the presence of the object and cause an alarm to be triggered.
Such anti-masking system increase the expense of the device, and in some circumstances are ineffective, because it is still possible to mask all or part of the Fresnel lens associated with the passive infra red sensor without traversing the light beam from the infra red LED. Thus a skilful thief can mask the lens without activating the anti-masking system.
U.S. Pat. No. 4,833,450 discloses an event detection which the alarm is sounded if a signal from a masking circuit exceeds a threshold level. The alarm continues to sound for a predetermined period. Once the predetermined period has lapsed the correct of operation of the event detection device is confirmed, the alarm is reset.
It is an object of embodiments to at least mitigate some of the problems of the prior art.